Roof area = plan area (including overhangs) × pitch factor, where pitch factor = √(1 + (rise/run)²). A 40 × 30 ft house with 12 in overhangs has 1,344 ft² of plan; at 6/12 (factor 1.118) the roof is 1,503 ft², or 16.5 squares and 50 bundles of shingles with 10% waste.
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Roof area incl. wastage
1,652.9ft²
Roofing squares (100 ft²)
16.53 sq
Shingle bundles (3 per square)
50 bundles
Rafter length (gable, eave to ridge)
17.89 ft
Roof angle
26.6 °
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Generated with myconstructioncalculator.com. Estimates for planning and ordering only; verify against drawings and specifications.
Formula
Plan area = (L + 2 × overhang) × (W + 2 × overhang)Pitch factor = √(1 + (rise ÷ run)²)Roof area = Plan area × pitch factor × (1 + wastage %)Squares = Roof area (ft²) ÷ 100 Bundles = Squares × 3
Worked Example
A 40 × 30 ft house, 12 in overhang, 6/12 pitch, 10% waste:
Plan = 42 × 32 = 1,344 ft²
Pitch factor = √(1 + 0.5²) = 1.118
Roof area = 1,344 × 1.118 = 1,502.6 ft²; with 10% = 1,652.9 ft²
Squares = 16.53; bundles = ⌈16.53 × 3⌉ = 50
Gable rafter (eave to ridge) = 16 × 1.118 = 17.89 ft
Pitch Factors
Pitch
Angle
Factor
Pitch
Angle
Factor
2/12
9.5°
1.014
7/12
30.3°
1.158
3/12
14.0°
1.031
8/12
33.7°
1.202
4/12
18.4°
1.054
9/12
36.9°
1.250
5/12
22.6°
1.083
10/12
39.8°
1.302
6/12
26.6°
1.118
12/12
45.0°
1.414
In metric, the factor is 1 ÷ cos(angle): 15° = 1.035, 20° = 1.064, 25° = 1.103, 30° = 1.155, 35° = 1.221.
Assumptions
All roof planes have the same pitch; overhang is the same on every side.
Rafter length is horizontal half-span × pitch factor, measured along the top of the rafter. It ignores ridge board thickness and tail cuts.
Starter strips, ridge caps and underlayment are not included.
Frequently Asked Questions
How do I calculate roof area from pitch?
Measure the building footprint including eave and gable overhangs, then multiply by the pitch factor √(1 + (rise ÷ run)²). For a 6/12 roof the factor is 1.118.
What is a roofing square?
100 square feet of roof surface. Three-tab and most architectural shingles come in 3 bundles per square.
Does a hip roof have more area than a gable roof?
Not if every face has the same pitch and the same overhang: the area is plan area × pitch factor in both cases. Hips and valleys do create more cutting waste, so use 15% wastage instead of 10%.
How does roof pitch change the roof area?
Roof area = plan area × √(1 + (rise ÷ run)²). A 6/12 pitch has a factor of 1.118, so a 1,344 ft² plan becomes 1,503 ft² of roof. Steeper roofs add more: a 12/12 (45°) roof is 41% larger than its plan.
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